The present invention relates to an application unit for directly or indirectly applying a fluid or pasty medium to a continuous material web, particularly one made of paper or cardboard. The invention also relates to a special method of adjusting a longitudinal profile and/or cross section of a fluid or pasty medium directly or indirectly applied by such an application unit to the continuous material web.
An application unit according to the class is known from WO 93/05887 and comprises a support beam which extends in its longitudinal direction substantially across the entire width of the material web, a counter-roll opposite the support beam, a doctor element, in the form of a doctor blade, detachably secured via holding means to the support beam and extending substantially across the entire width of the counter-roll, and a pressing member disposed on the support beam and to which a pressing force is directly and/or indirectly applied by pressing means, and with which pressing member, the doctor blade in the region of its doctor blade tip mated with the counter-roll can be pressed against the counter-roll at a predetermined pressure. A thin shank angled by the upper end of a two-armed lever mechanism, which transfers the force of the pressing means designed as a pressure hose and disposed at the lower end of the lever mechanism, serves as a pressing member in this known prior art; this shank extends substantially at a right angle to the doctor blade when the application unit is viewed in cross section. The entire pressing mechanism primarily serves to adjust the longitudinal profile, i.e. the layer thickness of the applied fluid or pasty medium.
This structural design is, however, extremely complex and expensive and also results in various disadvantages when setting the doctor blade or the longitudinal profile. In consequence, the pressing member's arrangement and design not only frequently result in a deformation of the pressing member itself at higher pressures and hence in a change of the force vector originating from the pressing means, thereby producing a resultant adjustment error at the doctor blade, but the lever mechanism's adjusting movement necessary for altering the contact pressure also results in a not inconsiderable deviation of the contact point's position between the pressing member and doctor blade. This in turn results in an undesirable deflection or deformation of the doctor blade, an increase or decrease in the contact pressure and a change in the doctor blade's coating angle, thus impinging on the application's coating quality. Every time the doctor blade is adjusted by the pressing member, the usually non-linear dependency of doctor blade load and coating angle therefore causes a changing application weight and hence fluctuating coating qualities. A relatively large friction between these two parts also results from the aforementioned shift in contact point between pressing member and doctor blade, which conditions comparatively large adjusting forces or large profiling forces when setting the cross section. Disadvantageous hysteresis effects also arise in such a design. These negative phenomena are particularly marked if the lever mechanism or the holding means to which the thin shank acting as a pressing member is attached has a thin and elastic design. Finally, because this embodiment has a complex overall pressing mechanism design, such an embodiment has also proved to be highly prone to dirt accumulation and to entail complex cleaning and maintenance.
Another application unit comparable with the aforementioned prior art is shown in EP 0 512 971 A1. In this known embodiment, the pressing means is itself designed as the pressing member that makes direct contact with the doctor blade and to which the pressing force is applied. Two pressure hoses, which are superposed in the vertical direction when viewing the application unit in cross section at that side of the doctor blade facing away from the counter-roll, and which exert a contact force on the doctor blade, are used as pressing means or pressing members. This solution does, however, result in a very substantial deflection of the doctor blade and hence simultaneously results in an undesirable change in the doctor blade's coating angle when the contact pressure is increased.